In the same way as with a radioimmunoassay, the principle of the radioreceptor assay is based on the biospecific detection of a ligand (used in a planned manner for the assay), e.g. a hormone, pharmacon, neurotransmitter, etc. at the corresponding target or acceptor molecule, e.g. an antibody in the radioimmunoassay or a receptor in the radioreceptor assay. As a result of the radioactive labelling of such a ligand used in a planned manner, an observable molecule is obtained, i.e. the tracer, which is in competitive interaction with an unlabelled ligand and therefore the similar, but unobservable, molecule. Through such measures a test system is obtained, which makes it possible to measure an unknown concentration of such a ligand.
While radioimmunoassay has become a widely used routine method and is used to a significant extent as an analytical method at present, radioreceptor assay is only rarely used for this purpose.
One of the main reasons for this is that the handling of biologically active receptors is more difficult than with an antibody. The prerequisite for a routine method is, apart from relatively simple handling (method simplicity) and the accompanying economical aspects, mainly the "stability" of the chemical or biological reactant. This means the stability of the actual analysis substance and its stability within the analytical reaction.
However, in solution, biologically active receptors are unstable and must therefore be frozen solid until just before they undergo analytical use. In addition, they are so unstable that even if this condition is briefly not fulfilled, the receptor can be made unuseable. It is obvious that such sensitive characteristics make it impossible to use as a routine method such a fundamentally useable analysis mechanism.